Clustering refers to using two or more systems that work together. It generally refers to multiple computer systems that are linked together in order to handle variable workloads or to provide continued operation in the event one system fails. Each computer may be a multiprocessor system itself. For example, a cluster of four computers, each with four central processing units (CPUs), would provide a total of 16 CPUs processing simultaneously. Another cluster system may include multiple server systems in a rack configuration where each server may have multiple processor circuit boards wherein each processor circuit board has multiple processors. These cluster systems often require substantial cabling for power and also provisions for cooling. For example, while all processors in the servers may be run for short times to handle processing requirements, the generated power density may require selected processors to be throttled or shut down or put in a low power mode to prevent thermal overloads. Cluster systems are becoming popular as a way of providing very high peak service for customers, therefore requiring methods to deal with the resulting power distribution problems. Strategies for managing power distribution problems in cluster systems encompass, varying performance of individual processors, partitioning cooling units, etc.
In a cluster of servers where power consumption is important, persistent connections and persistent states may defeat cluster-wide power management policies. A persistent connection is a required link between processors or programs within a cluster system or between the cluster system and external client systems which must be maintained to accomplish a task. A persistent state is a state of a processor or a program that must be remembered or maintained for an operation within the cluster system to insure that if the operation is suspended or interrupted for any reason, resumption of the operation will pick up with its resources intact. An example of persistent connections occurs frequently in the World Wide Web (WWW) when a customer performs a purchase over the Internet. The customer starts by authenticating with the server and then invokes several operations on the server's Web site to handle the purchase and payment. During the transaction, the customer's authorized credentials must be kept in a session data structure at the server. This data needs to persist while the transaction is in progress to ensure privacy and security.
Modern cluster systems used in large database systems (e.g., Internet) are being configured into super-dense cluster systems where many processors are placed on a single processor board and many such multiple processor (MP) boards are housed in racks. This may create very high power density concerns. Many strategies are being proposed to have dynamic power distribution management to deal with the high power densities of MP servers. Some of these techniques advocate shutting down some processors or machines if the aggregate workload may not use the entire processing capacity of the cluster. However, it may not be possible to shut down a machine or a processor within the cluster if there is an executing long-running task. Also, if there are persistent links between the machine and other machines or clients, or if there is a persistent state information that exists in volatile or stable memory that must be maintained on behalf of clients. Having limitations posed by the requirements of persistent connections and states thus greatly reduces the possibilities for power management strategies. There is, therefore, a need for method to eliminate limitations of persistent connections and states to facilitate power management in high density servers.